It is commonly held that the doctrine of equivalents is lost when claim amendments are made during patent prosecution.  That is, any claim amendment that is made during patent prosecution surrenders or gives up elements which are equivalent to those elements claimed in the amendment.  Surrender of the doctrine of equivalents places the burden on the patent Applicant, and the patent practitioner representing the Applicant, to carefully consider any amendments to the claims prior to issuance of the patent (when such occurs).  This principle is neatly summed up in the rulings on the following two cases.

In re Zletz, 893 F.2d 319, 322, 13 USPQ2d 1320, 1322 (Fed. Cir. 1989) (manner of claim interpretation that is used by courts in litigation is not the manner of claim interpretation that is applicable during prosecution of a pending application before the PTO).  Sage Prods., Inc. v. Devon Indus., Inc., 126 F.3d 1420, 1425, 44 USPQ2d 1103, 1107 (Fed. Cir. 1997) (patentee who had a clear opportunity to negotiate broader claims during prosecution but did not do so, may not seek to expand the claims through the doctrine of equivalents, for it is the patentee, not the public, who must bear the cost of failure to seek protection for this foreseeable alteration of its claimed structure).

What does this mean for the patent practitioner?  The claims as originally filed in a non-provisional patent application have full use of the doctrine of equivalents.  In an infringement proceeding, any claim that is granted as originally filed covers not only the literal elements in the claims but the equivalents of those elements.  But, an amended claim loses the equivalents.  So, the patent practitioner should exercise caution and prudence when amending claims.  Usually, a claim is amended during patent prosecution to clarify the meaning of a claim element, or to narrow or broaden the extent of a claim element in light of art that is cited by the Examiner.  As the Sage ruling above warns us, we have the opportunity to negotiate broader claims during prosecution.  If we don’t take advantage of this opportunity to establish a desired breadth of any claim element amended, we give up that broader breadth.

Some examples may illuminate.  In an electronics patent application, suppose the claim as originally filed recites MOS transistors.  But, the specification teaches that bipolar transistors could also be used in a circuit, as could relays, etc.  If the claim as originally filed is granted, it may be possible to argue that the claim covers other types of transistors and also relays and so on, as these are equivalent to the MOS transistors recited in the claim.  If, during prosecution, the claims are amended to recite NMOS transistors (e.g., because some of the art cites a PMOS transistor embodiment and there is good reason to argue that an NMOS embodiment is not shown in the art), then the claims only cover NMOS transistors.  It will not be possible to argue that the claim covers other types of transistors and also relays, because these equivalents have been surrendered in the amendment.

In a mechanical patent application, suppose the claim as originally filed recites threaded fasteners.  But, the specification teaches that nails, welding, rivets, etc. could also be used to hold something together.  If the claim is amended to recite nuts and bolts, instead of threaded fasteners, then screws, which are not nuts and bolts, and nails and rivets, etc. are surrendered through loss of the equivalents.

Now, supposing in each of these cases, there are other arguments to be made that differentiate the claims from the art, and these other arguments don’t depend on the narrowing as described above.  If the patent practitioner carelessly leaves the above amendments in place, the equivalents are surrendered needlessly, and the issued patent is narrower than was necessary in light of the cited art.  This situation is to be avoided.  What should a practitioner do?

When other arguments are found successful against cited art, one strategy is to revisit previous amendments and carefully consider whether a broadening amendment could then be made so that the literal equivalents are captured in the claims.  In the electronics example, suppose an amendment is made elsewhere in the claim, and the limitations regarding NMOS transistors are amended to recite transistors, or better still, switches (provided the specification explains that various types of transistors, relays, etc. can be used as switches).  Now, the amended claim element recites switches, and has literal coverage for the types of equivalents the Applicant intends.  In the mechanical example, suppose an amendment is made elsewhere in the claim, and the limitations regarding nuts and bolts are amended to recite fasteners, or attachment, or some other broader term, or a list of devices that fasten or attach two materials together.  Now, the amended claim recites the literal coverage for the types of equivalents the Applicant intends.  This is taking advantage of the opportunity that the above cases suggest is before us.  Failing to take advantage of this opportunity may have dire costs for the Applicant, in terms of lost claims coverage.  Of course, art cited by the Examiner should be considered, as the above tactic of broadening may reintroduce art earlier eliminated by the initial narrowing amendment and arguments.  The broadening tactic is successful only if the claims, as amended as a whole, do not read on any of the art under 35 USC §102 and 35 USC §103.

There are also more subtle aspects to this.  Comparison language such as less than or equal to, greater than or equal to, thresholds and limits may also be subject to the loss of equivalents in amendment.  Words and phrases such as “or”, “and”, “one of” versus “at least one of” or “one or more of” may be subject to strict scrutiny during Markman hearings, and the doctrine of equivalents may have bearing on the scope of the claims.  Watch these carefully, and amend these prudently.  The doctrine of equivalents, and loss thereof through amendments, is a recommended awareness to have in the art of patenting.